This invention relates to a process for producing an optical fiber by a modified CVD method (MCVD method).
Heretofore, known is a process for producing an optical fiber which has the steps of inner-lining a quartz tube with synthetic glass having a refractive index higher than the quartz tube to form a portion, ordinarily called "a core" and, wire drawing it via a wire-drawing furnace while simultaneously collapsing it. For example, the conventional process is disclosed in U.S. Pat. No. 3,711,262. The prior invention utilizes the steps of heating a quartz tube at a high temperature at wire-drawing time to soften it and contracting it by its surface tension, thereby providing an advantage that the production efficiency is high due to the simultaneous steps of wire-drawing and collapsing it.
However, according to the conventional process, the heating conditions required for collapsing it and the heating conditions necessary for wire-drawing do not always coincide. As a result, there arise various drawbacks that the central portion of the optical fiber is not solidified, so-called "an incomplete collapsing" or its circularity in case of solidifying is deteriorated to cause it to be deformed so that the collapsing step becomes unstable. Further, when the heating conditions of collapsing is preferentially prepared, another drawback that the wire-drawing conditions are restricted, takes place.
In addition, the quartz tube made by forming a core in the tube as described above is hollow in the center, and moisture in the atmosphere is mixed in case of wire-drawing, the moisture is introduced into the optical fiber at the time of wire-drawing to cause the transmission loss of the optical fiber to increase.